How can to do the inverse of explode modifier in Blender
You may do it using the Video Sequence Editor.


Go to the Video Sequence Editor window (Sequencer) and add the scene animation strip to it (Shift+A-->Scene-->Scene). Check the Backwards checkbox in the Filter panel of the properties shelf (press N being in the Video Sequence Editor window). 


Press the Image Preview button to see the preview of your animation. Render animation with F12.